Update newrelic.ini config to use heroku config vars

Using https://devcenter.heroku.com/articles/newrelic#node-js-agent-on-heroku as a guide, this seems like the thing to do.

Additional new relic should be put in code and should use the extra.heroku.php-includes option in composer.json
Merge pull request #60 from hedinfaok/patch-3

Update newrelic.ini config to use heroku config vars